In related reprogramming, a personal computer (PC), serving as a writing tool, and a vehicle-mounted control device (ECU: engine control unit) are connected via a controller area network (CAN) which operates in slow speed and a load module (a new program) is divided and transmitted to be written in a flash memory in the ECU.
Further, also in a case that a part of the new program to be updated in the older version of the program is small, the entire new program is transferred via the CAN and the entire new program is written.
Thus, there has been a problem that writing takes time. To address this problem, a concept of differential reprogramming has been proposed (PTL 1, for example). In other words, in paragraph 0020 in PTL 1, difference rewriting is described as one of rewriting method.